The compact and portable electronic devices such as PDA (personal digital assistant) or cellulous phone are essential for mobile communication era. The compact and portable electronic devices generally uses foldable keyboard for inputting data. The foldable keyboard should be compact for portability while the operational convenience should be preserved.
However, the conventional foldable keyboard for portable electronic devices such as PDA still has a volume larger than PDA itself after the foldable keyboard is folded. More particularly, the conventional foldable keyboard has considerable thickness after the foldable keyboard is folded. It is not convenient for user to put the folded keyboard in his pocket for carry. Moreover, the conventional foldable keyboard has complicated components; the weight and size of the conventional foldable keyboard are hard to be shrunk.
FIGS. 1 and 2 show a conventional foldable keyboard 1a, which comprises two outer panels 10a and 11a, and two inner panels 12a and 13a. The foldable keyboard 1a further comprises a plurality of keys 14a arranged on the two outer panels 10a and 11a, and the two inner panels 12a and 13a. The two inner panels 12a and 13a are pivotally connected and the backsides thereof face to each other when the two inner panels 12a and 13a are collapsed. Moreover, the two outer panels 10a and 11a are pivotally connected to the two inner panels 12a and 13a through respective hinge unit 15a. Therefore, the two outer panels 10a and 11a can be collapsed with respect to the two inner panels 12a and 13a, respectively such that the two inner panels 12a and 13a are sandwiched between the two outer panels 10a and 11a. 
The above-mentioned foldable keyboard 1a is segmented to four foldable segments such that the foldable keyboard 1a has only one-fourth area after it is folded. However, the thickness is excessively large and makes the foldable keyboard 1a hard to be put in user's pocket.
Moreover, the key 14a in the above-mentioned foldable keyboard 1a may have scissor structure (not shown) to reduce the thickness of the foldable keyboard 1a after the foldable keyboard 1a is folded. However, the scissor structure is complicated and increases weight of the foldable keyboard 1a. 
Alternatively, the spacing between adjacent keys can be decrease to reduce the volume of the folded keyboard. However, the volume reduction is limited and the reduced spacing may increase probability of typing error.